Which Javascript engine would you embed in your application? [closed]

I want to embed Javascript in a hobby game engine of mine. Now that we have the 5th generation of Javascript engines out (all blazing fast) I'm curious what engine would you choose to embed in a C++ framework (that includes actual ease of embeding it)?

^{Note: Just to make it clear, I'm not interested in DOM scripting or writing Javascript in a browser.}

Here's a compilation of links so far and some tips from the thread

• SpiderMonkey
• tracemonkey (note:backwards compatible with spidermonkey):
• V8
• Squirrelfish

Just for the record, I love Lua and have already embedded it in game engines about 5 times at work.

However now this is a hobby project, and I think that Javascript being known by most web developers and because its ECMA, Flash and Flex developers, a game engine that uses Javascript and XML for scripting would be more user-friendly and cater to a larger user base (and one that so far has not had a chance to use their skills for games) than one with Lua (and there are plenty of those around!).

Also for the record I'll go with V8 on this one, mostly because I like it's C++ style.

I've tried both SpiderMonkey and V8. With SpiderMonkey, I couldn't get anything to work. I couldn't even get the examples on mozilla.org to compile.

V8 worked out-of-the-box and I got some basic C++ <-> Javascript interaction going pretty quickly. There are some google lists for people using V8, and I found most of my questions answered there already.

Mozilla's SpiderMonkey is fairly easy and well-documented. It's a C API, but it's straightforward to wrap it in C++. It can be compiled to be thread-safe, which is useful for games since you'd likely want to have your main logic in one thread and user interface logic in a second thread.

Google's V8 might be a good choice, since you're using C++, but I have no experience with it yet. According to the documentation (thanks to Daniel James), V8 is not thread-safe, although this may change in the future.

There's also WebKit's SquirrelFish, but I couldn't find a standalone version of that when I was looking earlier.

I believe that v8 only works on x86, x64 and arm processors at the moment. Which might be a disadvantage.

With regards to thread safety, from include/v8.h:

 * Multiple threads in V8 are allowed, but only one thread at a time
 * is allowed to use V8.  The definition of 'using V8' includes
 * accessing handles or holding onto object pointers obtained from V8
 * handles.  It is up to the user of V8 to ensure (perhaps with
 * locking) that this constraint is not violated.

You can read more in the source file (it looks like doxygen documentation, but they don't seem to have put it up anywhere).

Update: That comment has been removed, probably some time ago. It looks like v8 now has an Isolate object which represents an instance of the engine. A single Isolate instance can only be used in a single thread at a time, but other Isolate instances can be used in other threads at the same time.

Is Java Script really the right language for your game? Many of games out there are using the Lua programming language for scripting. It's easy to integrate, it's very small, it compiles on almost every platform and it's easy to learn.

This somewhat off topic, but thinking outside the box can be important to get things right .